ringcentral java sdk RestClient authorize throws null pointer exception

Running following code on tomcat throws null pointer exception. Running same code in java main method runs fine.

        RestClient rc = new RestClient(clientID, clientSecret, serverURL);
        System.out.println("server-> " + rc.server);
        try {
            rc.authorize(username, extension, password);
            GetVersionsResponse result = rc.restapi().list();
            System.out.println("Result " + result.uri);
            rc.revoke();
        }
        catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        catch (RestException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

java.lang.NullPointerException

at com.ringcentral.RestClient.requestRaw(RestClient.java:257)

at com.ringcentral.RestClient.request(RestClient.java:320)

at com.ringcentral.RestClient.request(RestClient.java:251)

at com.ringcentral.RestClient.post(RestClient.java:146)

at com.ringcentral.paths.restapi.oauth.token.Index.post(Index.kt:18)

at com.ringcentral.RestClient.authorize(RestClient.java:113)